Simply, you need to buy a Balloon Anal plug. You insert it and the inflate the small balloon and it prevents the plug from coming out. IN the BDSM world these are often used by a master on a slave before she goes to work or before she goes out of the house, that way she has to wear the plug full time and it reminds her if her submission to her master AND to remove it, she would have to deflate it and then the master would know (it can only be re-inflated with the pump and hose) and would rightly punish her for it.
I don't have any, but I've seen places as docile as Amazon.com sell these in sets of three's, representing three sizes. That should solve your problem. Of course the one with the bulb would be a solution as well.